As the complexity of integrated circuits continues to increase, there is a continuing need to reduce dimensions of the individual components of the integrated circuits to increase device packing density. For field effect transistors, this need includes not only reducing the dimensions of the channel but also the areas of the source and drain regions.
One approach to reducing the channel length is described in U.S. Pat. No. 4,324,038 issued on Apr. 13, 1982 to C. C. Chang et al. The device disclosed can be termed a "grooved gate" which is formed by patterning a metal or silicide, silicon oxide, silicon nitride structure to form an opening between the field oxide regions, depositing and etching an oxide layer to form insulating sidewall spacers, growing a gate oxide and making the appropriate metal contacts. In forming the sidewalls, the silicon nitride acts as an etch stop to prevent overetching of the silicon oxide. The sidewalls act as an insulator to prevent contact between the gate metal and the source and drain contact layers. The described gate structure may be adversely affected by the etching which is necessary for the gate, because the etching may damage the portion of the substrate forming the channel. Additionally, patterning requires a gate head to compensate for misregistration and the horizontal dimension required for the gate layout is not a minimum.
It has also been realized that self-aligned source and drain contacts can reduce device size. One such process is described by NEC in Japanese Journal of Applied Physics, 21, pp. 34-38, Symposium on VLSI Technology, Sept. 1-3, 1982. After gate formation and source and drain implantation, an oxide is formed which is thicker on the gate than on the bulk silicon. Etching removes the oxide on the bulk silicon while leaving oxide on the gate. A nitride layer is now formed on the bulk silicon and serves as a mask during a second oxidation. The nitride is removed and doped polysilicon deposited for the source and drain contacts. IBM Technical Disclosure Bulletin, 26, pp. 4303-4307, January, 1984 describes a generally similar process although the nitride layer is formed by ion implantation.
The NEC process suffers several drawbacks. It is difficult to control the oxide etching because there is no etch stop and reliance must be placed on the difference in oxide thicknesses. Additionally, growing the nitride at high temperature is not desirable because the junction may be driven deeper. The IBM process suffers the drawback that the ion implantation is likely to create many defects and lead to high stress levels.
Another approach to producing devices with small area source and drain regions is described in U.S. Pat. No. 4,453,306 issued on June 12, 1984 to William T. Lynch and Frederick Vratny. Device fabrication begins with formation of a multilevel gate electrode having sequential layers of a conductor, an insulator and a silicide forming metal deposited on an insulating layer. Standard depositing and patterning techniques are used. A layer of polycrystalline silicon is blanket deposited over the entire structure, and a heat treatment forms the silicide on the gate structure. A selective etch removes the silicide and leaves the polysilicon aligned with respect to both the source and drain regions. Windows are formed in a blanket deposited dielectric layer for contacts to both the source and drain regions.
Although the structure is self-aligned, the precise control of the process may be difficult as the boundary between the polysilicon and the silicide may not be sharp because diffusion and etching of the sidewalls may occur. Also , the etching of the four layer metal containing sandwich may require a separate etching machine because of possible contamination. The windows are spaced further from the gate electrode than are the source and drain regions thus reducing packing density. The spacing of the windows is stated as desirable because it, e.g., reduces the possibility of aluminum spiking into the substrate and simplifies etching as all three contacts are at approximately the same level. Deeper etching for source and drain contact window than for the gate might cause growth of the gate window.